For decades, servers and other types of electronic devices have been implemented with input/output (I/O) devices. One type of I/O device is a serial port, which is normally a 9-pin or 25-pin connector positioned along a physically accessible portion of the electronic device. Since data is normally processed in parallel within an electronic device, the serial port is adapted to convert data from a parallel format into a serial format. A serial universal asynchronous receiver transmitter (hereinafter referred to as a “UART”) normally performs this format conversion.
As shown in FIG. 1, a conventional multi-port serial line circuit 110 implemented within an electronic device is shown. The conventional serial line circuit 110 features a plurality of serial UARTs 1201-1208 as well as corresponding dual channel opto-isolators 1301-1308 and serial line drivers 1401-1408.
As shown, each serial UART 1201, . . . , or 1208 is configured to convert bytes received from internal circuitry of the electronic device into a serial bitstream. The serial bitstream is routed to corresponding serial line drivers 1401-1408, which buffer incoming data, as needed, and drive the data onto a serial interconnect coupled thereto.
In order to provide voltage isolation, a single opto-isolator 1301, . . . , or 1308 is positioned between a serial UART 1201-1208 and its corresponding serial line driver 1401-1408, respectively. An “opto-isolator” is a semiconductor device that allows signals to be transferred between circuits, but keeping those circuits electrically isolated from each other. This protects the circuitry from damage from electrical transients (e.g., electrical surges, ground potential differences, etc.).
Herein, each opto-isolator 1301, . . . , or 1308 comprises a diode (e.g., light-emitting diode “LED”, infrared-emitting diode “IRED” or laser diode) for signal transmission, and a photosensor for signal reception. The diode converts an electrical signal into a beam of visible, modulated light or infrared (IR). The beam crosses a transparent gap and is picked up by the photosensor. The photosensor converts the light beam or IR back into an output electrical signal. The output electrical signal is identical to the input electrical signal, although the input and output amplitudes may differ. Normally, the opto-isolator 1301, . . . , or 1308 is enclosed in a single package appearing as an integrated circuit or a canned transistor with extra leads.
This architecture poses a number of disadvantages, especially for electronic devices having multiple serial ports. For instance, one disadvantage is that the total costs incurred in supporting multiple serial ports, such as a 24-serial port terminal server for example, is substantial. For instance, 24 opto-isolators generally cost around $54 in today's market. These costs normally prompt manufacturers to discontinue using such circuits and provide consumers with inferior electronic devices, or such costs are passed on to consumers.
Another disadvantage is that these opto-isolators occupy a substantial amount of real estate when mounted on a circuit board. Thus, any reduction in the number of requisite opto-isolators would free board area and enable smaller electronic devices.
Yet another disadvantage is that these opto-isolators require power at the electrically isolated side, which is expensive to supply. Thus, any reduction in the number of opto-isolators would provide useful power and cost savings.